Modern files are made from steel containing a relatively high percentage of Carbon - about 1.1 -1.3%. This allows the steel to be hardened so that it will cut into most other metals including most other steels in use today. In former times other metals were used such as iron and bronze which were perfectly capable of working the bone, wood and softer metals which were common at that period. It wasn't until about 1740 that Benjamin Huntsman perfected the technique of making hardenable steel in Sheffield, UK, that the modern type of file was possible.
Several types of file and rasp exist, but I shall confine myself here to the most common type - the engineers' file.
In this type of file, a flat or rounded piece of steel has sharp teeth raised on its surface which will cut into the workpiece when the file is pushed over its surface. These teeth are formed by striking the surface of the file blank with a sharp steel chisel. Until almost the end of the 19th century, this was usually done by hand - a very labour intensive and tiring job when one considers that a single file can have up to 5000 teeth on it.
Here we can see the shape of the teeth cut by the chisel. When cut by machine, the spacing of the teeth is set automatically but in hand cutting this is governed by the force of the blow given to the chisel. After each cut the chisel is raised, moved onto the uncut metal (to the left in this illustration) and slid back along the work until it meets the back of the previous tooth. It is then struck again with the hammer to form the next tooth. Obviously, if the chisel is struck harder, it will cut a deeper groove which will also be correspondingly wider and so, for a file of even pitch and roughness, it was important that the hand file makers develop the skill to make every blow of the hammer the same.
To achieve maximum efficiency both in cutting and in keeping the file clear of the metal chips it produces, the teeth are cut in a very particular manner. On a typical flat file there are two set of teeth, the UPCUT and the OVERCUT. Basically, the UPCUT teeth are there to cut the metal while the OVERCUT teeth clear away the debris and these two sets of teeth cross each other almost at right angles as can be seen in the diagram at the top of this page. For some reason ALL files are made right handed and it is very difficult for a left handed person to file well. This is because the relative angles of the upcut and overcut together with a slight difference in the pitch of the teeth give a 'rowing' effect as shown by the red line in the diagram. This can be seen if a file is held at a shallow angle and you look along it from the handle end. Because of this, a file always cuts better if the handle is slightly to the right of the point which is difficult for the left handed person to achieve.
This work is Copyright to Ian W. Wright 2003. You may use it for your own private purposes but reproduction by any means or its use for commercial gain is strictly forbidden without the written permission of the author.